So I'm looking at building my first telescope. I read the post on the UT blog linking to the One Minute Astronomer tips for the frugal astronomer, which led me to online plans to make a 4.25" reflector. I have a little Celestron spotting scope, which I picked up after Tammy Plotner's good review of it on the Universe Today site, and I've been pretty happy with it, considering what it is. I figure I'm pretty handy, I'd like to build something a bit bigger, and the 4" Newtonian seems to fit my budget and be portable enough that I'd use it.

I've been looking for mirrors, as Edmund Scientific doesn't seem to carry the ones recommended in the plans anymore, and I've come across two mirror sets that fit what I'm looking for. Newport Glass' 4.25" F/10 kit, and Coulter Optical's 4.5" F/8 kit. Now I only have a very rudimentary understanding of all of this stuff, so I guess I'm here to ask what kind of differences I'm likely to expect between the two, or, more specifically, how much of a difference will there be, since it's not like I'm looking at a huge size difference or difference in focal length here. The Coulter mirrors will also be a bit cheaper, which is always nice.

Anyway, thanks in advance for any answers.

From my experience, there is very little difference between f/8 and f/10 other than the length of the optical tube. The longer focal length optics can tolerate some imperfections or misalignments without a lot of noticible effect compared to shorter tube scopes. Therefore I think they are suited to homebuilt telescopes. In my opinion, if the coatings, mirror thickness and figure (accuracy of the shape) are comparable then go with the cheaper F/8. (I assume they are both spherical rather than parabolic? I didn't look these up - I also assume these are not "grind your own" kits since these are unlikely to specify an F stop unless certain templates are included)

Good luck.

There'd be very little difference. The f/8 would give a bit less magnification with any given eyepiece. Other than that there's little difference. The eyepiece would require less bending to reach with the f/10. Both would be a back bender in a Dobsonian design (easiest to build and best for a beginner to build as well). Each would give about the same view.

BUT I'd certainly look to a 6" if at all possible. I consider the 4.5" a kids scope not really for adults as the eyepiece will be low and what you can do with it rather limited. It's hard to appreciate the view when bent at an awkward angle. A 6" gives twice the light grasp making faint things twice as bright and showing you about 4 times as many stars than the 4.5" and gives nearly twice the resolution as well. A globular will show as a ball of stars not a fuzzy blob as it does in the 4.5". While in theory the resolution is only about 50% better the added brightness makes fine detail easier to see so the impression is of a greater resolution increase. Cost is little more but what it will show greatly more. The 1200mm focal length of a 6" f/8 is well suited for the magnifications you'll most often use as well as they can be achieved with basic (inexpensive) 1.25" eyepieces. The cost of a 6" is only a few dollars more than the 4.5" ($50 in many cases) yet the gain in performance for dollar spent is far greater.

Today you can buy a scope for about the same cost or even less as making one. It's far more satisfying to view with one you made but that's starting with bare glass and grinding your mirror. It doesn't sound like you are looking at that option as you are looking at finished mirrors. Though Newport does sell mirror grinding kits, by the time you grind it, add the cost to get it coated, build a basic Dobsonian mount, a tube, a couple eyepieces, mirror cell, spider, secondary focuser and finder you'll be paying more than the cost of a similar scope from say Orion. If you are handy with tools and can build your own parts then you will save money. We all did that when I started over 50 years ago. Then there was a large difference in cost to build or buy, that isn't true today with cheap bur reasonable quality Asian made scopes available today.
http://www.telescope.com/control/pro...oduct_id=08942

I'd save a bit longer and go that route unless you have the help of a local club to guide you through the rough spots.

Rick

You're correct in assuming that they're not "grind your own" kits. I'd like to try that at some point, but I'm thinking that I'll get the hang of the basic construction first. Also, both are indeed spherical.

Thanks for the reply.

From a photographer's point of view, an f/8 vs. an f/10 would allow you to gather the same amount of light in 64% of the time. There may not be a big difference to your eye, but to a camera there is.

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"Why do something now when you can put it off 'til later?" -- me :)

I've been considering going the 6" route, but the extra cost is a bit prohibitive at this point, although I'm still poking around for different prices on mirrors. I'm intending to build most everything other than the mirrors and eyepieces myself. From plans I've found online, I should be able to handle myself well enough, as I'm pretty good with tools and construction. You make a good argument for spending the extra to go the 6" route, though.

Aside from the fact that I'm on a fairly tight budget, there's also a big appeal to building something myself, just because I generally like the idea of tinkering with things and working on my own stuff, and it would be a nice project.

For a given aperture this is true ONLY if the image scale changes accordingly. If the f/8 and f/10 work at the same power or image scale then there's no difference in exposure time. After over 50 years in astrophotography I consisider f ratio rather meaningless. Aperture is far more important. Once you have the light you can adjust image scale in many ways to achieve the scale needed for the object you are imaging. F ratio only changes the adjustments you make to do this.

Thus for imaging say the moon or Jupiter the f/10 will make getting the image scale wanted easier and the exposure time would be the same as with an f/8 or for that matter an f/2 scope.

For imaging say M42 at prime focus an f/8 scope using a 12 micron pixel CCD will have the same image scale as a similar aperture scope running at f/4 with 6 micron pixels. The image scale would be the same as would the exposure time assuming similar quantum efficiency CCDs were used.

Rick